1. Field of the Invention
The present invention relates to switching regulators that produce a desired output voltage from an input voltage.
2. Description of Related Art
FIG. 4 is a circuit diagram showing an example of the configuration of a conventional switching regulator.
The conventional switching regulator having the configuration shown in this figure compares a predetermined reference voltage Vref with a feedback voltage Vfb commensurate with an output voltage Vo, and, based on the resultant comparison signal Sa and a predetermined clock signal Sb, produces a pulse width modulation signal Sc (hereinafter the “PWM (pulse width modulation) signal Sc”) with a desired duty factor, and hence a gate signal Sd of an output transistor 101.
In the conventional switching regulator having the configuration shown in this figure, the output transistor 101 is turned on at the rising edge of the clock signal Sb, and is turned off at the falling edge of the comparison signal Sa.
Thus, from the source of the output transistor 101 having the drain to which an input voltage Vi is applied, a switching voltage Vsw in the form of a rectangular wave according to the on/off of the output transistor 101 is outputted. By smoothing the switching voltage Vsw, a desired output voltage Vo is produced.
Examples of conventional technologies related to what has been described thus far are seen in JP-A-2006-33958 (hereinafter “Patent Document 1”) and JP-A-2006-246626 (hereinafter “Patent Document 2”).
Patent Document 1 discloses and proposes a current-mode control type switching regulator that performs, to prevent a subharmonic oscillation, an appropriate amount of slope compensation by using the result of calculation performed on an input voltage and an output voltage.
Patent Document 2 discloses and proposes a current-mode control type switching regulator that performs, to prevent a subharmonic oscillation, an appropriate amount of slope compensation by using a voltage difference between an input voltage and an output voltage.
It is true that the conventional switching regulator having the configuration shown in FIG. 4 can produce a desired output voltage Vo from the input voltage Vi with a simple configuration.
However, in the conventional switching regulator configured as described above, there is a possibility that an inevitable delay developed in the operations of the comparator 102, the latch circuit 105, and the preliminary driver 106 makes an output operation unstable, or makes it impossible to obtain a desired oscillating frequency. In particular, the influence of such delay becomes apparent, and hence the above-described problems often arise, when an attempt is made to increase a switching frequency.
Conventionally, to solve the above-described problems, a speed-up capacitor C2 or the like is added externally. Doing so, however, unnecessarily increases the number of external devices, and thus increases costs.
Additionally, in the conventional switching regulator configured as described above, like the current-mode control type switching regulator, a subharmonic oscillation tends to occur when the ON duty of the output transistor 101 exceeds 50%, such as when a voltage difference between the input voltage Vi and the output voltage Vo is small (for example, output voltage Vo/input voltage Vi≧½). This unfavorably increases ripples, leading to a lower degree of output accuracy.